Electrochemical sensors are often used in the laboratory or in process measurement technology to determine and monitor analysis measurands, such as concentrations of certain substances, so-called analytes, or variables dependent upon them. Examples of electrochemical sensors are potentiometric sensors, capacitive sensors, and amperometric sensors.
Potentiometric sensors generally comprise a measuring and a reference half-cell, also called a measuring and a reference electrode respectively. The half-cells can be in the form of a single-rod measuring chain in a common glass or plastic housing; they can, however, also be designed as two separable half-cells having separate glass or plastic housings.
A pH sensor designed as a single-rod measuring chain, in which the measuring half-cell is designed as a pH glass electrode, can have a glass housing with two glass tubes arranged coaxially to one another, wherein the outer glass tube is at one end connected to the inner glass tube so that the outer glass tube is closed at this end. The inner glass tube is closed at this end with a pH-sensitive glass membrane and is used as measuring half-cell housing. The chamber formed between the outer and the inner glass tubes is used as reference half-cell chamber.
The reference half-cell is often designed as an electrode of a second type, which has a reference element that is in contact with a reference electrolyte contained in the reference half-cell chamber. In the outer glass tube of the single-rod measuring chain is arranged a bridge that passes through the tube wall and that can comprise at least one diaphragm. The end region of the sensor that comprises the at least one diaphragm and the glass membrane is intended to be brought into contact with the measuring fluid. An exchange of substances, and thus an electrolytic connection between the reference electrolyte and the measuring fluid, is ensured via the diaphragm.
Such electrodes of the second type that comprise a reference element and an electrolyte in contact with the reference electrolyte, which electrolyte in turn is in contact with the measuring medium via a diaphragm, are also used as reference electrodes in other electrochemical sensors, such as in capacitive sensors that use capacitive field effect structures, for instance in the form of so-called EIS structures (“electrolyte insulator semiconductor”), to detect an analyte in a measuring solution.
Amperometric sensors comprise at least two, and often even three, electrodes. One of the electrodes is used as a working electrode, another as a counter electrode. A current flow between the working and the counter electrodes is used to determine the measurand. In many amperometric applications, the potential of the working electrode or the current flow through the working electrode is regulated to a constant value or a value that changes as a function of time by means of a third reference electrode through which no current flows. The electrodes are electrically conductively connected to a measuring circuit. Depending upon the type and measuring task of the amperometric sensor, the electrodes are immersed directly in the measuring medium or in an inner electrolyte accommodated in a housing. In the design mentioned last, the housing is closed by means of a membrane in a region provided for immersion in the measuring medium, through which membrane the analyte or a reaction product of the analyte can be transported into the inner electrolyte. As reference electrode, an electrode of the second type, for example, can be used that is designed in the same manner as the reference electrode of the previously described potentiometric sensors and thus also comprises a diaphragm used for the exchange of substances between the measuring fluid and the reference electrolyte.
Reference electrodes or reference half-cells can also be accommodated in plastic housings, rather than glass housings; this is, for example, often the case with ISFET sensors. Such a sensor is known from DE 10 2013 013601 A1, for example.
Diaphragms used as an electrolytic bridge of a reference electrode of an electrochemical sensor are often made from a porous ceramic body, such as a zirconium dioxide ceramic. The pores of the ceramic body form a connection, via which the reference half-cell chamber, in which the reference electrolyte is contained, and the surroundings of the housing of the reference half-cell communicate.
Depending upon the sensor type, diaphragms with different properties, such as different pore size and number of pores or pore density, are used. The pore size, pore density, number of pores, and the total cross-section that is provided by the pores and as a result of which a flow of electrolyte and/or measuring fluid is possible via the diaphragm, affect the measurement characteristics and/or the lifetime of the electrochemical sensor. Therefore, the diaphragms used also differ depending upon the specification of the sensor type. If sensors of a specific sensor type are produced only in relatively small quantities, only a small quantity of a ceramic specifically developed and produced for diaphragms of this sensor type is, accordingly, also required. The production and storage of respectively small quantities of specific ceramics are complex and costly. Ensuring, in a serial production, that each sensor type is respectively assigned the diaphragms adapted to it requires a relatively high logistical effort. An automation of the serial production is also complex, since it must be ensured that a device for the automated production of sensors is provided at any time with the diaphragms adapted to the sensor type to be produced at that time.